Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these.
The standardized IMS includes number of features that have relationships with each other. One of features is capability to share the same user identity in multiple devices as shown in this FIG. 1. For example, a user can have two or more devices that share the same identity for example the same IMS public user identity (IMPU), like public user identity 2 in FIG. 1. The shared identity can be used to initiate communication with any of these devices. Similarly a user can receive communication targeted to this identity with any of the devices bound to this identity. Identity concept could be extended also to cover use case where two or more different users, for example two family members, are sharing the same identity.
Voice call continuity (VCC) as defined by the 3GPP defines a capability to offer mobile voice services for a multi-radio terminal user via both circuit switched (CS) and packet switched (PS)/IMS access as preferred. The feature also contains capability to transfer ongoing call from one access to another e.g. from wireless local are network (WLAN) to CS access.
Single Radio VCC (SR-VCC) enables to perform handover from long term evolution (LTE) radio access network to CS access network. A network function is acting as a gateway between UE and an application server (AS) providing voice call continuity feature. The network function which can also be called as interworking function (IWF) can initiate a handover request towards the application server (AS) providing the voice call continuity based on received information from a radio access network, such as LTE, and from an evolved packet core system.
Multimedia session continuity enables continuity of PS media streams when PS access is changed, for example, by transferring a video session from WLAN to Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN) using IMS and application layer mechanisms. IMS centralized services (ICS) provide service execution in the IMS even though a device can use CS network to make calls. All services are executed in the IMS and all originated and terminated CS calls are forcedly routed to the IMS for service execution. To manage mid-call services ICS UE must use SIP signalling to initiate service execution. This control channel is called as I1-PS in ongoing standardization.
In FIG. 2, when two devices UE-1 and UE-2 share an identity, for example, “tel:123” and both devices are VCC capable and both have ongoing CS voice call which the network has anchored at the VCC application, it can happen that the UE-2 detects WLAN coverage and decides to perform a handover to WLAN. The UE-2 sends an INVITE request of SIP to a VCC Domain Transfer URI (VDI) and the request gets routed to the VCC application. The VDI is a SIP URI used by the UE to request a Domain Transfer Function (DTF) to perform a domain transfer to the IMS from the CS domain. However it is up to UE configuration which public identity the UE-2 set as a P-Preferred-Identity in the SIP request, and it may use an identity which is shared with UE-1, “tel:123”. Similarly if the identity is missing then the network can insert the default identity as A-party identity which can be “tel:123”. In either case, the VCC application is not able to differentiate between the CS bearers of the UE-1 and UE-2 to select which CS bearer is effected by the handover. In FIG. 3, similar problem can exist the other way as well when users have made calls via the IMS and then the UE-2 realizes that coverage is dropping and it must make a handover to CS. If the IMS originated calls are made with shared identity the VCC application is not able to see which IMS session should be effected.
In FIG. 4, with the IMS centralized services (ICS), for example, UE-1 has made a call via a CS access and can use I1-PS whenever needed. However, it is possible that UE-2 may make a call with the shared identity and the ICS AS must not associate this signalling with bearer signalling that UE1 is using. In ICS, the problem is more challenging if the dynamic I1-ps is used; there the UE-1 and UE-2 both may have an ongoing CS call (using the same identity in P-Asserted-Identity), and when either UE adds a I1-ps control channel using SIP, the AS must be able to determine to which CS call this SIP session is related. In FIG. 5, similar problem exist in SR-VCC. Starting point is the same as in above examples but in the SR-VCC, an IWF receives a request to start a handover and it generates a CS call towards a VCC Domain Transfer Number (VDN) and uses an identity available in CS side. The VDN is a public telecommunication number, used by the UE to request the DTF to perform a domain transfer to the CS domain from the IMS.
The object of the invention is to overcome the above problems.